The photocatalytic properties of the semiconductor material titanium dioxide result from the promotion of electrons from the valence band to the conduction band under the influence of ultraviolet (UV) and near-UV radiation. The reactive electron-hole pairs that are created migrate to the surface of the titanium dioxide particles where the holes oxidize adsorbed water to produce reactive hydroxyl radicals and the electrons reduce adsorbed oxygen to produce superoxide radicals, both of which can degrade NOx and volatile organic compounds (VOCs) in the air. In view of these properties, photocatalytic titanium dioxide has been employed in coatings and the like to remove pollutants from the air. Such coatings may also have the advantage of being self-cleaning since soil (grease, mildew, mold, algae, etc.) is also oxidized on the surface.
Despite the benefits of existing photocatalytic titanium dioxide coatings, there is room for improvement in the art. Particularly, it has been observed that the initial activity of conventional photocatalytic titanium dioxide coatings is poor unless the coating has been pre-activated, such as by washing with water. While not wishing to be bound by any theory, it is believed that the activation step is required to remove organic constituents present in the coating composition from the surface of the catalyst or possibly to provide a hydrated surface on the titanium dioxide particles from which reactive radical species are formed. However, this additional step makes application of a photocatalytic titanium dioxide coating somewhat inconvenient because it is time consuming and adds additional costs to the application process. It would be desirable to provide a photocatalytic titanium dioxide coating, particularly in the form of a paint, which does not require pre-activation (e.g., a washing step or exposure to elements) to achieve high initial activity levels.
It has also been difficult to provide coatings having high levels of photocatalyst because the catalyst tends to oxidize and break down the polymeric binder of the coating. This problem is exacerbated when the coating is exposed to intense UV radiation from direct sunlight, as is the case with an exterior paint. Such coatings are often formulated with inorganic binders or with organic polymers which are resistant to photocatalytic oxidation at relatively low catalyst concentrations. However, in low light conditions the de-pollution properties of the coating are less than optimal. It would be desirable to provide a coating for use in low light environments (e.g., indoors) that incorporates high levels of photocatalyst for optimal dc-pollution and which is resistant to degradation, yet provides high catalytic activity under indoor lighting conditions.
It is therefore an object of the present invention to provide coating compositions, particularly paint compositions, which comprise titanium dioxide photocatalysts capable of removing pollutants from the air, which photocatalysts have high initial activity without prior activation. It is a further object of the invention to provide durable coatings having high levels of photocatalytic titanium dioxide which coatings have de-pollution activity in low light environment, and in particular in the presence of visible light.
The foregoing discussion is presented solely to provide a better understanding of nature of the problems confronting the art and should not be construed in any way as an admission as to prior art nor should the citation of any reference herein be construed as an admission that such reference constitutes “prior art” to the instant application.